The quantity of steam entering a steam turbine which does not pass through and across the turbine buckets and which escapes to the atmosphere reduces the efficiency of the turbine. Therefore, to the maximum extent possible, there should not be any leakage of steam from the casing of a steam turbine before that steam has produced work. One or more split rings have been used to seal the interface between a steam turbine casing and the associated shaft. U.S. Pat. No. 3,503,616 to Hickey is one example of this concept. These ring seals are commonly used within a horizontally split housing or casing to enable the seals to be removed or replaced without removing the shaft and rotor. However, the upper casing must be removed or lifted to free the seal rings.
It is also known to provide a small opening in a steam turbine case to axially insert or remove small labyrinth seal segments through a special housing recess (i.e., U.S. Pat. No. 3,920,251 to Remberg). However, after these labyrinth steam seal segments have been axially located they must be then rotated to their final position by means of a special installation fixtures or tools. These methods are needlessly time consuming and inefficient. More importantly, the cost of modifying the turbine casing to receive these seal segments is high and replacement is not easy. Those skilled in the art also know that any seal scheme that requires careful machining of the turbine case is a seal scheme that is difficult to maintain and operate. It should also be clear that, in the case of an overhung steam turbine, axial thrust, and therefore thrust bearing or balance piston losses, can be minimized or reduced by directing the steam flow through a path away from the shaft bearings. It should be equally clear that rotor stability requirements restrict the space available for the insertion of a shaft seal between the rotor and the supporting bearings. In other words, the bearings for a turbine shaft should be placed as close as possible to the first stage rotor for that steam turbine. Thus, a difficult, and certainly a non-obvious, design problem is presented which requires solution.